This research proposal is designed to study how intracellular Na, Ca and H ion activities (iNa, iCa and iH) are regulated and how these intracellular ion activities are related to contractile force (T) and electrical potential (action potential and diastolic potential) in canine cardiac Purkinje fibers. A hypothesis to be tested is that the level of intracellular Na ion activity is an important factor in determining intracellular Ca level (by means of a Na-Ca exchange) and thereby contractile force. The proposal also intends to study characteristics of the Na-K pump and Na-H exchange in the muscle fibers. The intracellular ion activity, contractile force and electrical potential will be measured simultaneously and continuously in the fibers stimulated at a constant rate of 1 or 2 Hz. The intracellular Na, Ca and H ion activities will be measured with Na+, Ca++ and H+ selective microelectrodes respectively. Using these methods, it is planned: (1) to test whether norepinephrine and cyclic AMP stimulate the Na-K pump and decrease iNa; (2) to determine the relationships among iNa, iCa, T and electrical potential of the fibers exposed to low extracellular K+ concentrations and to a cardiac glycoside, digoxin and its antibody; (3) to determine the relationships among iH, iNa, T and electrical potential of the fibers when extracellular pH is changed and restored; (4) to determine the relationships among iNa, T and electrical potential of the fibers exposed to local anesthetics and tetrodotoxin, (5) to determine the relationships among iCa, iNa and T of the fibers exposed to Ca antagonists; (6) to determine the relative importance of ATP driven Ca-pump and Na-Ca exchange in the fibers. In the fibers exposed to the cardiac glycoside, we will also determine the relationships among iNa, iK (intracellular K ion activity), T and electrical potential. It is hoped that these experiments provide insights into the relation of intracellular ions to contractile force and the mechanism of cardiac glycoside action in heart muscle.